Method and apparatus to monitor quality of operation of a piston in a cylinder

ABSTRACT

A transducer is connected to the cylinder sleeve or liner, and deflection, preferably axial vibration or oscillation of the liner is converted into an electrical signal, which is integrated, and detected with respect to a reference level; if a vibration frequency, particularly in the ultrasonic range is detected, or if vibration amplitudes exceed a certain level, an alarm signal is provided. The reference level may be a dynamic level, in which output signals from all cylinders of a multicylinder engine are combined, and the combined signal is utilized as a dynamic reference for comparison with any individual cylinder signal. Additional operating parameters can be considered in a logic circuit, for example fuel supply, or extent of loading on the engine, with feedback to reduce fuel supply, or shut off the engine upon detection of excessive vibration in any one or more cylinders.

Unite Hirt et ai. ,Han. 8, 1974 [54] METHOD AND APPARATUS T0 MUNHTUR2,465,735 3/1949 Lieberherr ..73/l16 X QUALITY OF OPERATION OF A PISTONIN A CYLINDER Primary Examiner.lerry W. Myracle [75] Inventors: DieterHirt, Augsburg; Leonhard HQF'T'Q T F X91. FEQfQf Gerlich, Hirblingen;Eugen Heinie, Neusass, all of Germany [73] Assignee: Maschinenfabriir[57] ABSTRACT Augsburg'Nmmbelg A transducer is connected to the cylindersleeve or Aklllengeseuschafi, Augsburg, liner, and deflection,preferably axial vibration or os- Germany cillation of the liner isconverted into an electrical sig- [221 Filed: Jam 16 1973 nal, which isintegrated, and detected with respect to a reference level; if avibration frequency, particularly pp .1 ,111 in the ultrasonic range isdetected, or if vibration amplitudes exceed a certain level, an alarmsignal is pro- [30] Foreign Application Priority Data vided. Thereference level may be a dynamic level, in J 22 1972 G P 22 03 0473which output signals from all cylinders of a multiermany cylinder engineare combined, and the combined signal is utilized as a dynamic referencefor comparison g 2 5 with any individual cylinder signal. Additionaloperat- 58] g i l 1 9 R ing parameters can be considered in a logiccircuit, for

"""""""" 73/67 example fuel supply, or extent of loading on the engine,with feedback to reduce fuel supply, or shut off the engine upondetection of excessive vibration in [56] uNlTE g q xfigs gjgENTs any oneor more cylinders.

2,192,863 3/1940 Hetzel et al 73/1 16 X 21 Claims, 3 Drawing FiguresTRANSDUCER INTEGRATOR P a TH Z l R DETECTOR 9 13 1I.A| ARM B\ FREQUENCYSELECTIVE AMP.

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U P m METHOD AND APPARATUS T MDNIITDR QUALITY OF OPERATION OF A IPIISTUNTN A CYLINDER The present invention relates to an apparatus and to amethod to monitor the smoothness of operation of a piston reciprocatingwithin a cylinder, and more particularly the piston of a high speedinternal combustion engine. The output derived from a sensing element isan electrical quantity which can be utilized as an alarm signal, a stopsignal, or as a control signal to control supply of fuel, or otheroperation parameters of the engine.

Internal combustion engines, and particularly high power, internalcombustion engines are subject to damage, if minor defects at therunning surface of the piston (or the piston rings) within the cylinder(or, more particularly the cylinder sleeve) are present. Minor defectscan rapidly lead to freezing, or blocking of the piston within thecylinder, or to substantial scoring of the cylinder and the piston lineror piston sleeve, which, in extreme cases, may completely destroy theinternal combustion engine or, at least, the respective cylinderthereof.

It has previously been proposed to monitor the operation of internalcombustion engines by means of a temperature sensing device (see, forexample, German Pat. No. 742,697) which is so located that jamming of apiston in a cylinder is sensed, by excessive temperature rise, in orderto prevent locking, or excessive damage. As soon as a piston begins toscore the interior of a cylinder, the temperature of the piston (or thecylinder) will rise rapidly due to the substantially increased frictionin operation of the piston within the cylinder. A temperature sensingelement,,located in the piston, is so arranged that at a predeterminedtemperature it will have a predetermined expansion, or bend, whichtriggers an alarm signal when a limiting or'threshold temperature isexceeded. The alarm signal is then utilized to stop the engine. Thisarrangement does not operate well since the temperature will rise onlyat a time I when it is too late to prevent substantial damage to theengine by stopping the engine, or disabling a specific cylinder.

It is an object of the present invention to provide a monitoring methodand apparatus which indicates quickly that the piston is not operatingsmoothly, as desired, the indication being obtained in sufficient timeto prevent damage to the piston, or cylinder, or both.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, it has been found,surprisingly, that the amplitude, or the frequency (or both) ofoscillations of the cylindersleeve or cylinder liner can be utilized toprovide a suitable sensing parameter. The cylinder sleeve or cylinderliner is subject to some vibration or oscilla: tion and excessivevibration or oscillation amplitudes, or frequencies (or both) provide anindication that the operation of the piston within the cylinder is notas desired. It is thus possible to indicate faulty operation at itsinitial phase, so that corrective steps can be taken before substantialdamage is done. Usually, if incorrect running is detected quickly,defects can be repaired easily and without substantial effort or time.Sometimes it is merely sufficient to decrease the load on the engine.The pistons may then, at low load, run in properly again, so that theengine can be loaded to its limit thereafter, without outside repairbeing necessary. Such self correcting behavior of the engine must,however, be undertaken at an early stage of faulty operation.

The longitudinal vibrations or oscillations of the cylinder sleeve orcylinder liner form, in accordance with a preferred feature of theinvention, the measuring parameter. The amplitudes or frequency, orboth, of these vibrations or oscillations are sensed. The longitudinalvibrations of the cylinder sleeve rapidly increase at an early stage ofimproper operation, which, if permitted to continue, would lead toscoring or eventual binding of the pistons within the cylinder. Thus,the measuring instruments may be sturdy and need not be of delicate,very high sensitivity type. Such sturdy measuring instruments themselvesimprove the reliability of the entire system and the danger that themeasuring system, itself, is subject to breakdown is thus substantiallyavoided.

The preferred measuring parameter would be an electric voltage orcurrent. Electronic apparatus, which is readily available and can beconstructed as integrated circuits, can then be used to evaluateelectrical or voltage pulses, such electronic apparatus being simple andof extremely small size. Measuring and indication can be obtainedremotely, after amplification of the electrical parameters utilized as asensing element, so that indication devices can be located convenientlyto operating personnel and remote from interfering equipment or devices.

in accordance with a feature of the invention, the electrical signal isselectively amplified with respect to frequency, since the vibrationsarising as the running conditions deteriorate do not cause vibrationfrequencies throughout a wide frequency band, but rather cause vibrationto rise within a quite narrow band. Thus, frequency selectiveamplification is a good indication of deterioration of operating qualityof the engine. Vibrations within the ultrasonic region are particularlyprevalent; further, the inherent or critical vibrations of the cylindersleeve may become pronounced, and of high intensity, as the operatingconditions continue to deteriorate. Selecting vibrations therefore bothwith respect to frequency and with respect to amplirude (or, either one)permits rapid detection of changes in operating quality of the engine.

The invention will be described by way of example with reference to theaccompanying drawings wherein:

FIG. 1a is a schematic general circuit diagram of a monitoring systemfor a single cylinder;

FlG. llb is a fragmentary detailed view, to a greatly enlarged scale, ofa transducer arrangement for a single cylinder; and

FIG. 2 is a schematic block circuit diagram of a monitoring system for amulticylinder engine.

A piston P is operable longitudinally within the cylinder C, the insideof which is lined with a cylinder sleeve or cylinder liner l. The pistonhas piston rings 2. Excessive friction between the piston rings, or thepiston and the cylinder sleeve or liner may lead to hot spots orexcessive heating at particular regions of the piston, or on thecylinder; the temperature may even reach melting temperature of thematerials and in any event leads to substantial increased shear forcesacting on the cylinder liner l. The shear forces, due to friction, whichif permitted to persist may cause scoring or binding of the piston willintroduce longitudinal vibration and oscillation of the cylindersleeve 1. The amplitudes and the frequency (or either) of theseoscillations are sensed by a transducer 3 which provides electricaloutput signals in the form of voltage or current pulses. The signalsprovided by transducers 3 are amplified in the frequency selectiveamplifier 4 and connected to a rectifier system 5, in which positive andnegative pulses are rectified. Transducer 3 and amplifier 4 may beincluded in a single element located in a bore within the cylinder liner1 (see FIG. 1b). Usually, one single transducer per cylinder suffices.More than one transducer per cylinder may be used, the varioustransducers per cylinder being distributed about the circumference ofthe cylinder liner 1. Noise is essentially prevented from beingconducted to the frequency selective amplifier 4 by locating thetransducer directly on, or in the cylinder sleeve, so that vibration, orother extraneously arising disturbances can be eliminated, althoughdefective and uneven operation of the piston within the cylinder isdirectly sensed at the point where the relative movement (piston withinthe cylinder sleeve) occurs, without introduction of stray signals, orextraneous damping. Thus, the various parts can be easily assembled, andthe assembly step of the transducer can be carried out even before thecylinder sleeve is assembled into the cylinder block itself. Extraneousdisturbances will not affect the signals derived from the transducer. itis also possible to sense oscillations of the cylinder sleeve, forexample over solid state transmitting elements, such as solid statesound transmission devices leading to vibration sensors, or the like,which are located outside of the cylinder, or of the motor itself.External vibration sensing elements, sensing vibrations transmittedthereto by means of sound transmission elements are particularly simplein construction.

Rectifier 5 is connected to an integrator 6 which integrates the varioussingle pulses derived from transducer 3 over amplifier 4 and rectifier5, for a predetermined time, for example for the duration of oneoperating cycle of the engine. The integration period or interval iscontrolled by a control circuit including capacitor 8 and transistor 7,typically a field effect transistor (FET). The voltage on capacitor 8 isextinguished or discharged briefly once for each operating cycle, sothat the integration by integrator 6 will always start from the samestarting level. The integrator 6, at the end of any one integrationperiod thus will provide a final value which is independent of rotationor speed of the engine. This simple value is transferred to a samplingand holding circuit 9 (hereinafter: SH Ckt), to be stored therein. Thestorage of the result of the integration provides continuous,uninterrupted supervision of the operation of the engine. The outputfrom the SH Ckt 9 will, continuously, have the final integration valueappear thereat. Transistor 7 is controlled with respect to the SH Cktwith a slight delay, so that the output from the integrator is reliablytransferred to the circuit 9 from integrator 6. The timing pulses tocontrol integrator 6 and the SH Ckt 9 are derived from a wave shaping orpulse shaping circuit 10 which, in turn, is controlled by a transducer11 which provides one pulse for each rotation of the engine. The pulsescan readily be obtained from the crankshaft or from the camshaft of theengine, by connecting a cam to a cam-operated switch. If the signalobtained from the SH Ckt 9 exceeds a limit or level set by a referencelevel setting circuit 12, a threshold detector 13 will respond to effectgeneration of an alarm signal in alarm circuit 14.

The output from the sampling and hold circuit 9 is available at theterminal 9n. Multicylinder engines can be monitored by utilizing signalsfrom a number of cylinders, as will be explained in connection with FIG.2. In this figure, summing amplifiers are labeled S, reference levelgenerators L, and threshold detectors T. The reference level generatorsmay, for example, be put in the circuit by otentiometers.

The outputs from the various SH-Ckts 9 are applied to terminals 90, 9b9h corresponding to terminal 9n of FIG. 1. The signals at terminals 9a9h are connected to a summing amplifier 15 and, simultaneously, to ascanning switch and scan position indicator 16, which may also be termeda multiplexer. The signals, sequentially derived from scanning switch 16are applied to three threshold detectors. Threshold detector 16 comparesthe signal appearing at any one of the terminals 9a-9h with an averagevalue, forming an arithmetic average, of all the output signals of thevarious respective SH Ckts. The threshold circuit 17 compares therespective signal values from the scanning switch, derived from therespective SH Ckts with the arithmetic average of all the signals, toprovide a dynamic reference level. The signals, summed in summingamplifier 15 are increased by an additional level by adding a referencelevel from generator 19 in summing amplifier 18. If the individual valueof any output at terminal 90, 9b 9h exceeds the arithmetic average andadditional reference level from generator 19, then threshold detector 17will provide an output signal which can be utilized to operate the alarmdevice 20 to provide a preliminary alarm, or the like. Simultaneously,the specific cylinder from which the signal is derived can be indicatedon the position indicator portion of scanning switch and scanningposition indicator 16.

Scoring or uneven operation of the pistons arises usually only at heavyloading of the engine. The present system can be readily adapted toconsider additional criteria of engine operation in order to monitor thequality of operation of the motor, when the decision is to be madewhether the motor is to be stopped, or the loading thereon reduced. Thesignal of threshold detector 17 is, to this end, additionally applied toa logic circuit 23 which logically determines the decision. A referencelevel generator 22 is connected to a threshold detector 21 whichcompares the reference level from generator 22 with the individualsignal level at any one of the terminals 9a, 9b 9h. If these signals,applied sequentially, exceed the reference level from generator 22, thena signal is provided from threshold detector 21. Asignal representativeof an additional engine operating parameter is connected to logiccircuit 23, so that the logic circuit 23 will have a plurality ofsignals applied thereto which are decoded or analyzed so that a decisionbasis is provided to control the motor to reduce load or stop the motorentirely. Transducer 24' provides a fuel-supply signal. The output fromlogic circuit 23, available at line 23', is utilized to control the fuelsupply controller 24, associated with a fuel pump 25. Upon a decision inlogic network 23 that damage to the cylinder sleeve is imminent, assensed by excessive vibration, the output on line 23 is utilized tocontrol the fuel supply to the engine, over pump 25, to reduce theamount of fuel, or shut it off entirely. Other parameters canadditionally be introduced to the logic network, to further affect thedecision to be made therein, for example engine temperature or the like.If the internal combustion engine is to be operated with a high degreeof smoothness, so that even minor vibrations are analyzed and used tocontrol reduction in loading, then reduction of fuel supply to theengine can of course be controlled by a lesser number of inputs to logic23.

A summing amplifier 27 is connected to the output of summing amplifier15, and has additionally a negative reference level applied thereto fromgenerator 26, so that summing amplifier 27 will operate as subtraction,or difference circuit. The average value, that is,

the dynamic level of the various inputs from terminals 9a, 9b 9h,derived from summing amplifier is subtracted from a fixed referencelevel from generator 28; the remainder is compared with the respectiveindividual signals derived from the scanning switch 16 in thresholddetector 26. If the output from summing amplifier (or, rather,subtraction circuit) 27 falls by a predetermined value below the dynamicthreshold, as determined by threshold detector 26, a separate alarm 29is operated. This circuit is utilized to monitor proper operation ofoutput from the respective transducer 3 (FIG. la) providing the outputsignals, and the associated circuitry and components which form thevarious measuring channels having outputs connected to terminals 901, 9b9h. The various electronic circuits can be formed as integratingelements, located on small silicon chips, to provide reliable circuitcomponents, with low space requirements, and low power consumption.

The rectifier 5 (FIG. la) is not strictly necessary but is a preferredform; integrating both the positive and negative pulses derived fromtransducer 3 results in rapid evaluation of the outputs from thetransducer, and elimination of noise and statistically arisinginterference, and noise signals, to provide a clearly defined anddifferentiated and clearly recognizable warning signal.

The frequency of pulses derived from transducer 3 are not uniformlydistributed over a wide frequency spectrum, as the quality of operationof the engine deteriorates. Thus, amplifier 4 is a frequency selectiveamplifier and provides preferred and selective amplification of thosefrequencies which are expected to occur in particularly defined bands,so that the quality of operation can be closely monitored, and anydeterioration in smoothness of operation can be recognized quickly. Ithas been found by experience that vibrations and oscillations inultrasonic range become pronounced as soon as the quality of operationbegins to deteriorate, and before it has reached dangerous proportions.As the quality of operation continues to deteriorate, and scoring of thecylinder sleeve may become imminent, the inherent or criticalfrequencies, in longieffectively eliminated, and any one cylinder whichmight be damaged can be easily indicated. The arithmetic average, oranother median, average value derived from the summed output of summingamplifier 15 provides a dynamic, load-dependent reference value, withwhich the operating quality of the respective cylinders can be compared.The threshold value set by the reference level generator 19 preferablyis adjusted to be slightly above the noise level of the variouscylinders. This effectively prevents false alarms due to possible noisearising in the transducer 3 or the associated circuits of any particularchannel. The average or median value is also utilized for fail-safemonitoring of the operation of the various transducers and theirassociated channels by threshold detector 26 which compares the outputof the various channels with a decreased average value.

Scoring by the pistons, usually, occurs only at a certain operatingtemperature of the cylinder sleeve, or, independently of temperature,when the motor is quite heavily loaded, for example above percent ofrated full load. Since such vibrations frequently only occur at suchheavy loading, generation of an alarm signal can be utilizedsimultaneously to reduce the fuel supplied to the engine, or to stop theengine entirely. The logic circuit 23 (FIG. 2) can also be set in such amanner that only if the engine is subjected to a predetermined minimumload, for example 75 percent of rated full load, will the circuit beenergized or enabled to respond. This can readily be instrumented byusing AND gates. Preventing operation of the the circuit at low loads,or loads below a certain level, additionally insures against falsealarms.

The transducers themselves are preferably located at the end of thecylinder sleeve or cylinder liner which is not secured to the cylinderblock. This increases their sensitivity and improves the accuracy ofmeasurement, since the excursion, and the vibration or oscillation, ofthe end of the cylinder sleeve which is not secured to the cylinderblock is quite high and essentially occurs without damping.

FIG. lb illustrates one way in which a transducer can be located in thecylinder liner. A small hole is bored in one side of the cylinder linerll, closed off by a plug, for example a screw plug 106. A weight 103 isloacted within the bore, spaced centrally by means of a pair of springs104, 165. The weight includes a sensing coil, located opposite a face ofthe cylinder liner which, if made of iron, is magnetized asschematically indicated by N, S; otherwise, a small permanent magnet canbe inserted in the cylinder liner. Theweight 103 additionally can carryall the circuitry involved, if it is formed as an integrated circuit.Electrical connections are carried out by wires W7, passing through plug106. The entire assembly can be pre-assembled in the cylinder linerbefore the cylinder liner itself is secured to the engine, that iswithin the cylinder block C. Upon vibration of the cylinder liner 1,weight 103 will remain essentially stationary (with respect to thecylinder liner 11) due to its own inertia, and the magnetized portion NSof the cylinder liner will thus induce voltages, depending on the rateof change of magnetic flux through a coil, schematically indicated, andcarried on the weight W3. The induced pulses provide the output from thetransducer which is applied to the amplifier 4 and rectitier 5, both ofwhich can be included as a chipintegrated circuit on or in the weight103. The fuel control, and the application of the transducer to thecylinder liner have been described specifically in connection withdiesel engines; the invention is applicable, however, to anypiston-cylinder engine in which the cylinder has a cylinder sleeve orliner.

Various changes and modifications may be made, and features described inconnection with any combination or drawing may be applied to othercombinations within the inventive concept.

The type of transducer explained in connection with FIG. 1b need not beused, and other types of transducers are suitable. For example, apiezo-electric element, secured beneath a support mass which appliespressure against the piezo-electric element may be used, the supportmass being resiliently retained, for example under spring pressure, in abore of the cylinder sleeve or liner 1. Of course, an integrated circuitcan readily be secured within the same bore, adjacent the piezoelectricelement. The transducer system is loaded transversely, and is subjectedto single-ended compression. This construction is essentiallyinsensitive to deformations of the housing, temperature changes, andacoustic as well as magnetic interfering influences; it is alsoessentially immune against transverse oscillations. Such types oftransducers are made, for example, by the firm of Briiel & Kjaer, ofDenmark. Quartz is a suitable material for the piezo-electric element,and provides rapid response necessary in a transducer used in high-speedinternal combustion engines.

We claim:

1. Method of monitoring quality of operation, for example runningsmoothness, of a piston (P) in a cylinder (C), the cylinder having acylinder sleeve (1) in which an output signal representative of pistonoperation is obtained,

comprising the steps of sensing the deflection of the cylinder sleeve(1) with respect to the cylinder (C) as the piston (P) operates withinthe cylinder sleeve (1);

and deriving said output signal as a representation of said senseddeflection.

2. Method according to claim 1 including the step of analyzing theoutput signal for amplitude and generating an indication signalrepresentative of amplitudes exceeding a predetermined level.

3. Method according to claim 1 including the step of analyzing theoutput signal for a predetermined frequency range, and generating anindication signal representative of frequency within said range.

4. Method according to claim 3 wherein the frequency range is in theultrasonic range.

5. Method according to claim 1 including the step of analyzing theoutput signal for amplitude in excess of a certain range, and forfrequency within a predetermined band and generating an indicationsignal representative of either amplitude in excess of a predeterminedlevel or frequency within a predetermined range, or both.

6. Method according to claim 1 wherein the step of sensing deflection ofthe cylinder sleeve (1) comprises the step of sensing deflection of thecylinder sleeve in axial direction.

7. Method according to claim 1 wherein the output signal is electrical.

8. Method according to claim 1 further comprising the step of analyzingthe output signal with respect to predetermined frequencies, andamplifying said predetermined frequencies.

9. Method according to claim 1 in which the output signal has negativeand positive components;

and said method comprises the step of rectifying said components andintegrating the rectified components.

10. Method according to claim 9 wherein the integrating time constant isessentially the same as the time of one revolution of the engine pistoncrankshaft.

11. Method according to claim 10 including the step of a. storing theintegrated signal during one integration cycle until the end of asubsequent operation cycle of the piston, and storing the representationof the integrated signal;

b. erasing the integrated result;

c. and repeating steps (a) and (b).

12. Method to monitor a multi-cylinder engine comprising the steps ofindividually monitoring the respective cylinders of the engine inaccordance with the method of claim 1;

and analyzing the output signals derived from the individual cylinders.

13. Method according to claim 12 including the steps of forming anaverage value of derived output signals from all the cylinders toprovide a dynamic reference value signal;

and determining deviation of any individual output signal from saiddynamic reference with respect to a predetermined level.

14. Method according to claim 13 further comprising the step of adding asignal representative approximately of signal noise level arising duringthe sensing and deriving steps, in any one cylinder, to said dynamicreference level to form said predetermined level.

15. Method according to claim 13 further comprising the step ofindicating decrease of the dynamic reference level below a predeterminedlevel.

16. Method according to claim 15 wherein the step of indicating drop ofthe dynamic reference level comprises the step of forming the dilTerencebetween a signal representative of a predetermined level from thedynamic reference level signal;

and providing an indication if the remaining difference signal dropsbelow a further predetermined level.

17. Method according to claim 12 further comprising the step ofgenerating a signal representative of loading on the engine;

logically combining said loading signal with the individual derivedoutput signals from the cylinder; and controlling fuel supply to theengine in accordance with a characteristic of the logically combinedloading signal and cylinder output signals.

18. Apparatus to monitor the quality of operation of a piston (P) in acylinder (C), having a cylinder sleeve (1) to obtain an output signalrepresentative of quality of piston operation, comprising a transducerelement (3) located in vibration transfer relation to the cylindersleeve (1) or liner;

means deriving from said transducer an electrical output signal having acharacteristic representative of vibration of the cylinder sleeve, orliner;

and means (4, 5, 6, 9, l2, l3) analyzing said signal with reference to apredetermined level (12) and deriving an alarm output (14) if the signalexceeds said predetermined level.

19. Apparatus according to claim 18 wherein the transducer element (3;.103) is hermetically sealed in the cylinder sleeve (1).

20. Apparatus to monitor the operation of a multicylinder enginecomprising a plurality of apparatus according to claim 18, one

for each cylinder;

means (115) combining the output signals from the transducers to providea combined output signal forming a dynamic reference signal;

and means (17) comparing the combined dynamic reference signal with thesignals derived from the put signal.

1. Method of monitoring quality of operation, for example runningsmoothness, of a piston (P) in a cylinder (C), the cylinder having acylinder sleeve (1) in which an output signal representative of pistonoperation is obtained, comprising the steps of sensing the deflection ofthe cylinder sleeve (1) with respect to the cylinder (C) as the piston(P) operates within the cylinder sleeve (1); and deriving said outputsignal as a representation of said sensed deflection.
 2. Methodaccording to claim 1 including the step of analyzing the output signalfor amplitude and generating an indication signal representative ofamplitudes exceeding a predetermined level.
 3. Method according to claim1 including the step of analyzing the output signal for a predeterminedfrequency range, and generating an indication signal representative offrequency within said range.
 4. Method according to claim 3 wherein thefrequency range is in the ultrasonic range.
 5. Method according to claim1 including the step of analyzing the output signal for amplitude inexcess of a certain range, and for frequency within a predetermined bandand generating an indication signal representative of either amplitudein excess of a predetermined level or frequency within a predeterminedrange, or both.
 6. Method according to claim 1 wherein the step ofsensing deflection of the cylinder sleeve (1) comprises the step ofsensing deflection of the cylinder sleeve in axial direction.
 7. Methodaccording to claim 1 wherein the output signal is electrical.
 8. Methodaccording to claim 1 further comprising the step of analyzing the outputsignal with respect to predetermined frequencies, and amplifying saidpredetermined frequencies.
 9. Method according to claim 1 in which theoutput signal has negative and positive components; and said methodcomprises the step of rectifying said components and integrating therectified components.
 10. Method according to claim 9 wherein theintegrating time constant is essentially the same as the time of onerevolution of the engine piston crankshaft.
 11. Method according toclaim 10 including the step of a. storing the integrated signal duringone integration cycle until the end of a subsequent operation cycle ofthe piston, and storing the representation of the integrated signal; b.erasing the integrated result; c. and repeating steps (a) and (b). 12.Method to monitor a multi-cylinder engine comprising the steps ofindividually monitoring the respective cylinders of the engine inaccordance with the method of claim 1; and analyzing the output signalsderived from the individual cylinders.
 13. Method according to claim 12including the steps of forming an average value of derived outputsignals from all the cylinders to provide a dynamic reference valuesignal; and determining deviation of any individual output signal fromsaid dynamic reference with respect to a predetermined level.
 14. Methodaccording to claim 13 further comprising the step of adding a signalrepresentative approximately of signal noise level arising during thesensing and deriving steps, in any one cylinder, to said dynamicreference level to form said predetermined level.
 15. Method accordingto claim 13 further comprising the step of indicating decrease of thedynamic reference level below a predetermined level.
 16. Methodaccording to claim 15 wherein the step of indicating drop of the dynamicreference level comprises the step of forming the difference between asignal representative of a predetermined level from the dynamicreference level signal; and providing an indication if the remainingdifference signal drops below a further predetermined level.
 17. Methodaccording to claim 12 further comprising the step of generating a signalrepresentative of loading on the engine; logically combining saidloading signal with the individual derived output signals from thecylinder; and controlling fuel supply to the engine in accordance with acharacteristic of the logically combined loading signal and cylinderoutput signals.
 18. Apparatus to monitor the quality of operation of apiston (P) in a cylinder (C), having a cylinder sleeve (1) to obtain anoutput signal representative of quality of piston operation, comprisinga transducer element (3) located in vibration transfer relation to thecylinder sleeve (1) or liner; means deriving from said transducer anelectrical output signal having a characteristic representative ofvibration of the cylinder sleeve, or liner; and means (4, 5, 6, 9, 12,13) analyzing said signal with reference to a predetermined level (12)and deriving an alarm output (14) if the signal exceeds saidpredetermined level.
 19. Apparatus according to claim 18 wherein thetransducer element (3; 103) is hermetically sealed in the cylindersleeve (1).
 20. Apparatus to monitor the operation of a multi-cylinderengine comprising a plurality of apparatus according to claim 18, onefor each cylinder; means (15) combining the output signals from thetransducers to provide a combined output signal forming a dynamicreference signal; and means (17) comparing the combined dynamicreference signal with the signals derived from the indidual transducersof the individual cylinders to provide an alarm output if the signalfrom any one individual cylinder deviates from the dynamic referencesignal by a predetermined amount.
 21. Apparatus according to claim 20comprising means (24'') generating an engine operation signalrepresentative of an engine operating parameter; logic means (23)logically combining said dynamic reference signal, said individualoutput signals from the individual cylinders, and said operation signal,and deriving a control output signal; and feedback circuit means (23'',25) controlling engine operation under command of said control outputsignal.